Our power supply chokes are designed to work at 100/120Hz and with voltages around 100V. Most induction meters work at a few volts or less, at around 1kHz. This is not the correct operating point for a power supply choke, and the reading on the induction meter is usually around half the specified value of the choke.

Best way to measure the inductance in a power supply choke is to connect the choke to something like 100V, 50/60Hz, measure voltage and current and calculate inductance. I use a variable autotransformer connected to my mains outlet for this measurement. Be careful however how you connect the autotransformer to the mains, so you do not touch 220V when you think your terminal is at 0V.

An audio line is a cable transferring an audio signal from the output of one device to the input of another. Normally the input impedance is fairly high, >1k for mic preamps and >10k for line inputs.
If the impedances in both ends of an audio line are high, any noise current picked up by the cable will be easily audible, as the current will generate a significant voltage across the input resistor. But if one side is low impedance, the noise current will be short circuited and no voltage will be generated.
Even if the impedance of your output amplifier is fairly low, the use of an LL1540 will add almost 3k to the output impedance, which in most situations is far too much. In addition to the noise problem above, you will also get a certain signal loss across the LL1540, in particular if the input impedance of the next device is less than 10k.
A better choice is one of our general purpose transformers (LL1527, LL1588) or a dedicated line output transformer such as LL1517 or LL1585.

What is the sound difference between mu metal and amorphous core transformers? admin
2015-09-16T12:47:07+00:00

Many customers ask us for the difference in sound between our mu metal lamination transformers and our strip core amorphous transformers. We have forwarded the question to Kevin Carter at K&K Audio. Here are his impressions:

“When I first encountered the Lundahl LL1544A and LL1545A line level transformers, which use the same coils, but these are installed on cores made of different materials, I knew that it was a great opportunity to understand the role that these core materials play in “transformer sound” without being confused by other variables in transformer construction. A few years later, I was able to do the same sort of comparative listening in becoming familiar with the differences and similarities between identical coils assembled with different coil materials in the LL1931 and the LL1933 MC Input transformers. I found sonic consistency between the transformers using the same core material and differences between transformers of the same application type with different core materials. Here is a summary of my observations:
The amorphous core transformers (LL1544A and LL1931) provide a very open and detailed “picture” of the space that was recorded, providing a very high level of detail recovery. A particularly memorable example of this “see-through” quality was evident on a solo violin recording where the performer’s breathing was readily audible during playback of the performance with the amorphous core transformers, but was only evident with the mu-metal core transformers if the listener really focused on hearing the breathing. The breathing sounds were there with both transformers, but there is less of an emphasis on detail and more upper bass / lower midrange warmth with the mu-metal transformers (LL1545A and LL1933). The brush strokes are broader with the mu-metal core transformers and they convey more “body”, whereas the amorphous core transformers offer finer brush strokes, which results in hearing more of the details while listening to the whole.

The difference between the transformers made with the two different core materials is not large, but the detail and tonal shading differences are easy to hear with the appropriate recordings. If the equipment in which one of these transformers were to be installed with has plenty of sonic body already then I would choose the amorphous core version for its substantial transparency. On the other hand, if the sound is a bit light-weight, then a mu-metal version is probably the best way to go.”

LL1660s Data Sheet Confusion. How come an LL1660S/10mA is used at 18mA? admin
2015-06-23T12:34:18+00:00

If we assume a certain DC current, there are two ways to reach a desired core operating point:
1. With a given coil, adjust the core air-gap or
2. With a given core air-gap, adjust the number of turns on the coil

The type number LL1660S/18mA is designed such that at 18mA DC current and ALL PRIMARY WINDINGS CONNECTED IN SERIES, the core operating point is 0.9T But if you connect the windings in a different way (such as suggested in “Alt B”), the number of magnetizing turns will be different, and the core will have a different operating point unless you change the DC current.

So for LL1660S/10mA, used in connection “Alt B”, the necessary DC current required to reach the operating point 0.9T is 18mA

LL5801 is a 1+1 : 2 audio isolation transformer originally designed for Kungliga Dramatiska Teatern (Royal Theater) in Stockholm. Sorry, no data sheet. A modern replacemen is LL1527 or LL1588. LL5801 pinout and usage:

I’ve built an amp with 2A3 and LL1660. I like the sound of it,
but was a little frustrated when I measured the frequency response
today. Around 9.5KHz I get a climbing response
that doesn’t go away until I’ve reached the end of the bandwidth of the
amp. With a few measurements, I located the problem to be the 1660. It’s
wired in the ALT-T hookup, to allow 20mA current through the 6H30.

Is this expected? If not (and also if so, I suppose :-), what can I do
about this deviation?

Hi Per,

Seems I already got this explained to me by an audioasylum inmate, it’s
an impedance problem. So I’ve got something to work with now, sorry for
wasting your time.In case you want to know, 24K resistance from the grid of the 2A3 to ground made the frequency response curve perfectly flat. I can’t wait to listen to it and see how this changed the sonics of the amp. BTW, thanks a lot for making quality products for tube amps. I already absolutely love the sound of my new amp.
–
Jens Axboe

I was measuring the Inductance of the primary coil of the LL1649. It measures around 2-3 H with the primary coil in series. In general what is the optimum required H value for the primary. Why do you want to measure the inductance of a mains transformer? admin
2015-05-28T14:07:24+00:00

The interesting parameter for a mains transformer is the no-load current. For our mains transformers LL1648 — 51 the no load current is less than 100mA, indicating a no load impedance above 2kohms at 50 Hz. (corresponding to 6H) The no-load current should be compared to the full-load current, which for a 250VA transformer is around 1A. A 1:10 factor between no-load current and full load current is in my opinion quite acceptable.

One of the LL1674 seems to be off-spec, the secondary static resistance should be about 605 ohm and 3 ot the transformers do indeed measure 600-608 ohm. The fourth one measures 560 and 600 ohm for the two seconday resistances, this would lead to an off-balance surely? admin
2015-05-28T14:06:03+00:00

Gain matching and CMR would not be so good.
Difference in copper resistance is due to difference in wire gauge. It is possible that we used wire from different runs or from different vendors. However, the number of turns should be correct. (I say ”should be” as shit do happens, even at Lundahls. But if the number of turns was not correct, the difference in copper resistance should have been around 100 ohms for the LL1674 (representing two full layers).)

Which problem will arise? The difference in DC resistance of the transformer should be compared to the impedance of the load. If the load impedance is 20k + 20k, (reasonable or even low load for the LL1674), the 40 ohms difference of the transformer is in the magnitude of 1/1000 of the load impedance. In my opinion, this is ignorable. Besides, is the core degaussed after measuring DC resistance with a DC current? This is probably a bigger problem.

What is the difference between LL1570XL & LL1581XL? admin
2015-05-28T14:04:47+00:00

The internal structures of the LL1581XL and LL1570XL are quite different. The LL1581XL is a splitting transformer with very high immunity to external noise and noise crosstalk. When used as a splitting transformer, the LL1570XL may pick up electromagnetic hum (from e.g. motrs or power supplies).

I am building a mic preamp based on the 990 amp block. The 990 likes to see 600 ohms.I have heard the 1538xl and was wondering if that will work or if you have any other suggestions? admin
2015-05-28T14:04:07+00:00

Why not parallel LL9206 high impedance windings? I use the LL9206 as a MC-stepup with turnratio 1:10, and I’m wondering why you don’t recommend termination ”D”. A lower copper resistance should give a better S/N-ratio, or do I miss something? admin
2015-05-28T14:01:24+00:00

In a step-up transformer the internal capacitance in high impedance windings are always high, due to the many turns of thin wire. In addition, the signal level across a high impedance winding is high (relatively) due to the step-up. When putting high impedance windings in parallel, the result is a high capacitive load on the primary, and dropping HF response. This is the reason why we generally recommend NOT to parallel high impedance windings.

Can I use any of your transformers for converting consumer level -10dB unbalanced signals to professional +4dB balanced signals? admin
2015-05-28T14:00:44+00:00

In order to lift signal level from -10 to +4 you need a stepup transformer with turns ratio 1:5, such as LL1935 used 1:5 The consumer equipment output impedance should be reasonable low, and pro audio input impedance must be high, as the transformed output impedance will be 25 times higher than the original source impedance. The transformer should be placed close to the balanced input. If the source impedance is much higher than 600 ohms, the band width will be limited.

Can I use an LL1671/30mA in a PP configuration? admin
2015-05-28T13:59:43+00:00

Yes, you can use our SE transformers in PP applications. The drawback is that due to the DC-accepting core airgap in the SE transformer, transformer inductance is less than for corresponding PP version.

The primary impedance of most of your transformers are not listed in the data sheets. Why? admin
2015-05-28T13:51:20+00:00

The primary impedance of a transfomer in an application, as seen by the source, is the primary no load impedance in parallel with the transformed load. The ”primary no load impedance” for a gapped tube amp transformer is the primary inductance. The ”transformed load” is the secondary load as seen from the primary winding (Load * (turns ratio)*(turns ratio)). Thus, the transformer by itself does not have an impedance that can be used without a context.